The GIF file format is complex and has different non-image data that can be stored before and after an image data. The GIF metadata design allows you to read/write any GIF non-image data and prevents misunderstanding with GIF extensions storing order.
The following metadata are always saved before an image data:
- GIF Logical Screen Descriptor Level
- GIF Image Descriptor Level
- GIF Global Color Table Level
- GIF Local Color Table Level
The metadata of GIF extensions that are inside GIF Extensions After Image Level (between items with Name "AfterImageExtensions" and Id 0xFFF (Type LEVEL_START and LEVEL_END)) are saved after an image data.
All other GIF Extensions Metadata are saved before an image data.
Brief information on GIF metadata levels is provided in the set of tables below:
GIF Metadata Level
| Name |
Id |
Type |
Value Type |
Value |
Value Length |
Read Only |
| "GIF" |
IG_FORMAT_GIF |
LEVEL_START |
0 |
NULL |
0 |
TRUE |
| GIF header level |
| GIF Logical Screen Descriptor level |
| GIF Image Descriptor level |
| optional Global and/or Local Color Tables and optional GIF Extensions levels (extensions before image) |
| "GIF" |
IG_FORMAT_GIF |
LEVEL_END |
0 |
NULL |
0 |
TRUE |
All items between items with Name "GIF" and Id IG_FORMAT_GIF (Type LEVEL_START and LEVEL_END) are interpreted as GIF data. If during sending data from application level to filter level the first item is omitted the data will not be interpreted and saved.
GIF Header Level
| Name |
Id |
Type |
Value Type |
Value |
Value Length |
Read Only |
| "GIFHeader" |
0x10 |
LEVEL_START |
0 |
NULL |
0 |
TRUE |
| "Signature" |
0x101 |
VALUE_ITEM |
STRING |
"GIF" |
3 |
TRUE |
| "Version" |
0x102 |
VALUE_ITEM |
STRING |
|
3 |
TRUE |
| "GIFHeader" |
0x10 |
LEVEL_END |
0 |
NULL |
0 |
TRUE |
GIF Logical Screen Descriptor Level
| Name |
Id |
Type |
Value Type |
Value |
Value Length |
Read Only |
| "LogicalScreenDescriptor" |
0x20 |
LEVEL_START |
0 |
NULL |
0 |
TRUE |
| "LogicalScreenWidth" |
0x201 |
VALUE_ITEM |
UINT16 |
|
1 |
FALSE |
| "LogicalScreenHeight" |
0x202 |
VALUE_ITEM |
UINT16 |
|
1 |
FALSE |
| |
| "Fields" |
0x205 |
LEVEL_START |
0 |
NULL |
0 |
TRUE |
| "GlobalColorTableFlag" |
0x206 |
VALUE_ITEM |
BOOL |
|
1 |
FALSE |
| "ColorResolution" |
0x207 |
VALUE_ITEM |
UINT8 |
|
1 |
FALSE |
| "SortFlag" |
0x208 |
VALUE_ITEM |
BOOL |
|
1 |
FALSE |
| "GlobalColorTableSize" |
0x209 |
VALUE_ITEM |
UINT8 |
|
1 |
FALSE |
| "Fields" |
0x205 |
LEVEL_END |
0 |
NULL |
0 |
TRUE |
| |
| "BackgroundColorIndex" |
0x203 |
VALUE_ITEM |
UINT8 |
|
|
FALSE |
| "PixelAspectRatio" |
0x204 |
VALUE_ITEM |
UINT8 |
|
|
FALSE |
| "LogicalScreenDescriptor" |
0x20 |
LEVEL_END |
0 |
NULL |
0 |
TRUE |
GIF Global Color Table Level
| Name |
Id |
Type |
Value Type |
Value |
Value Length |
Read Only |
| "GlobalColorTable" |
0x30 |
VALUE_ITEM |
RAW_DATA |
|
Variable |
TRUE |
GIF Image Descriptor Level
| Name |
Id |
Type |
Value Type |
Value |
Value Length |
Read Only |
| "ImageDescriptor" |
0x2C |
LEVEL_START |
0 |
NULL |
0 |
TRUE |
| "ImageLeftPosi tion" |
0x2C1 |
VALUE_ITEM |
UINT16 |
|
1 |
FALSE |
| "ImageTopPosition" |
0x2C2 |
VALUE_ITEM |
UINT16 |
|
1 |
FALSE |
| "ImageWidth" |
0x2C3 |
VALUE_ITEM |
|
|
1 |
FALSE |
| "ImageHeight" |
0x2C4 |
VALUE_ITEM |
|
|
1 |
FALSE |
| |
| "Fields" |
0x2C5 |
LEVEL_START |
0 |
NULL |
0 |
TRUE |
| "LocalColorTableFlag" |
0x2C6 |
VALUE_ITEM |
BOOL |
|
1 |
FALSE |
| "InterlaceFlag" |
0x2C7 |
VALUE_ITEM |
BOOL |
|
1 |
FALSE |
| "SortFlag" |
0x2C8 |
VALUE_ITEM |
BOOL |
|
1 |
FALSE |
| "LocalColorTableSize" |
0x2C9 |
VALUE_ITEM |
UINT8 |
|
1 |
FALSE |
| "Fields" |
0x2C5 |
LEVEL_END |
0 |
NULL |
0 |
TRUE |
| |
| "ImageDescriptor" |
0x2C |
LEVEL_END |
0 |
NULL |
0 |
TRUE |
GIF Local Color Table Level
| Name |
Id |
Type |
Value Type |
Value |
Value Length |
Read Only |
| "LocalColorTable" |
0x40 |
VALUE_ITEM |
RAW_DATA |
|
Variable |
TRUE |
GIF Extensions Metadata
GIF Graphic Control Extension Level
| Name |
Id |
Type |
Value Type |
Value |
Value Length |
Read Only |
| "GraphicControlExtension" |
0xF9 |
LEVEL_START |
0 |
NULL |
0 |
TRUE |
| |
| "Fields" |
0xF91 |
LEVEL_START |
0 |
NULL |
0 |
TRUE |
| "DisposalMethod" |
0xF92 |
VALUE_ITEM |
UINT8 |
|
1 |
FALSE |
| "UserInputFlag" |
0xF93 |
VALUE_ITEM |
BOOL |
|
1 |
FALSE |
| "TransparentColorFlag" |
0xF94 |
VALUE_ITEM |
BOOL |
|
1 |
FALSE |
| "Fields" |
0xF91 |
LEVEL_END |
0 |
NULL |
0 |
TRUE |
| |
| "DelayTime" |
0xF95 |
VALUE_ITEM |
UINT16 |
|
1 |
FALSE |
| "TransparentColorIndex" |
0xF96 |
VALUE_ITEM |
UINT8 |
|
1 |
FALSE |
| "GraphicControlExtension" |
0xF9 |
LEVEL_END |
0 |
NULL |
0 |
TRUE |
GIF Comment Extension Level
| Name |
Id |
Type |
Value Type |
Value |
Value Length |
Read Only |
| "CommentExtension" |
0xFE |
VALUE_ITEM |
STRING |
|
Variable |
TRUE |
GIF Application Extension Level
| Name |
Id |
Type |
Value Type |
Value |
Value Length |
Read Only |
| "ApplicationExtension" |
0xFF |
LEVEL_START |
0 |
NULL |
0 |
TRUE |
| "ApplicationIdentifier" |
0xFF1 |
VALUE_ITEM |
STRING |
|
8 |
FALSE |
| "Appl.AuthenticationCode" |
0xFF2 |
VALUE_ITEM |
UINT8 |
|
4 |
FALSE |
| "ApplicationData" |
0xFF3 |
VALUE_ITEM |
RAW_DATA |
|
Variable |
FALSE |
| "ApplicationExtension" |
0xFF |
LEVEL_END |
0 |
NULL |
0 |
TRUE |
GIF Plain Text Extension Level
| Name |
Id |
Type |
Value Type |
Value |
Value Length |
Read Only |
| "PlainTextExtension" |
0x01 |
LEVEL_START |
0 |
NULL |
0 |
TRUE |
| "TextGridLeftPosition" |
0x11 |
VALUE_ITEM |
UINT16 |
|
1 |
FALSE |
| "TextGridTopPosition" |
0x12 |
VALUE_ITEM |
UINT16 |
|
1 |
FALSE |
| "TextGridWidth" |
0x13 |
VALUE_ITEM |
UINT16 |
|
1 |
FALSE |
| "TextGridHeight" |
0x14 |
VALUE_ITEM |
UINT16 |
|
1 |
FALSE |
| "CharacterCellWidth" |
0x15 |
VALUE_ITEM |
UINT8 |
|
1 |
FALSE |
| "CharacterCellHeight" |
0x16 |
VALUE_ITEM |
UINT8 |
|
1 |
FALSE |
| "TextForegroundColorIndex" |
0x17 |
VALUE_ITEM |
UINT8 |
|
1 |
FALSE |
| "TextBackgroundColorIndex" |
0x18 |
VALUE_ITEM |
UINT8 |
|
1 |
FALSE |
| "PlainTextData" |
0x19 |
VALUE_ITEM |
STRING |
|
Variable |
FALSE |
GIF Extensions After Image Level
| Name |
Id |
Type |
Value Type |
Value |
Value Length |
Read Only |
| "AfterImageExtensions" |
0xFFF |
LEVEL_START |
0 |
NULL |
0 |
TRUE |
| One or more GIF extension levels |
| "AfterImageExtensions" |
0xFFF |
LEVEL_END |
0 |
NULL |
0 |
TRUE |
Callback Required for Writing GIF Metadata Items
The GIF Logical Screen Descriptor Level and GIF Image Descriptor Level metadata items can be written using LPAFT_IG_METAD_ITEM_SET_CB callback function.
Other GIF metadata can be written using LPAFT_IG_METAD_ITEM_ADD_CB callback function.
GIF Metadata Item ID Constants
Please see file enumIGGIFTagIDs for the complete list of GIF Metadata Item Id constants.
